A leopard silently stalks its prey. Controlling every muscle in its body, it
inches its way towards an antelope oblivious to impending doom. Any sound will
give the leopard away, but this expert hunter is as careful as it is patient.
Slowly, the leopard moves closer to the antelope, until it stands just feet
away, ready to strike. The scene unfolds so slowly and dramatically it might
seem to you a nightmarish fantasy—one of the most remarkable exchanges in
nature. There is only one problem. You can't see a thing.

The leopard's enormous eyes are capable of vast dilation, which allows
the predator to see in near-lightless conditions. Human eyes, like the eyes of
the leopard's prey, become useless in low light. Until recently, this meant
that we had no way of observing the night behavior of leopards. Today,
infrared-sensitive cameras enable us to record on videotape what we are unable
to see with our eyes.

In order to understand how this is possible in total darkness, we must first
understand light. Light travels in waves, much like ripples on a pool. Colors
are simply light waves of different lengths. Very short light waves are violet
in color; as the waves get longer, the light changes to indigo, blue, green,
yellow, orange, and finally red. The waves can
even be measured. From one peak to the next, an indigo-colored light wave is
400 nanometers (400 billionths of a meter) and a red wave is 700 nanometers.
This range of colors is all that the human eye is capable of seeing—our
"visible light spectrum." But what happens when waves get even longer or
shorter?

Looking at a chart, one quickly realizes that the wavelengths we can see make
up a very small portion of the total spectrum. As the waves get longer than red, they become so-called
"near-infrared" waves, then infrared waves, microwaves, and lastly radio waves.
As they get shorter than indigo, they become ultraviolet waves, X rays, and
finally gamma waves. These waves surround us all the time, but we are unaware
of their presence, since they are not in our visible light spectrum.

Near-infrared waves behave much like the light waves that we can see. They
come in a range of lengths, and are absorbed and reflected by objects. But for
our purposes there is one critical difference: They go undetected by humans,
leopards, and antelopes. This is great news for producers who are looking to
capture the behavior of wildlife at night.

For the NOVA film Leopards of the Night, producers Amanda Barrett and
Owen Newman brought security cameras to Africa (see Behind the
Scenes). They were not paranoid that their equipment would be stolen.
Instead, they had their minds on stealing something for themselves: nighttime
video images. While our eyes cannot see waves longer than 700 nanometers,
certain security cameras are sensitive to waves well into the near-infrared
range.

In order for these cameras to record dramatic wildlife scenes, near-infrared
waves are needed to "illuminate" the subjects. Barrett and Newman relied on
special lights as a source for these necessary waves. These lights, called
infrared illuminators, use LEDs (light-emitting diodes) and are designed to
produce light of particular wavelengths. In this case, they emit waves between
800 and 900 nanometers, right in the near-infrared range. While no visible
light is emitted, the area is flooded with these invisible waves. The animals
remain totally unaware of this illumination and go about their business in the
dark. But as with visible light, their bodies absorb or reflect these
near-infrared waves, depending on what part of the animal the waves bounce off.

Focusing the infrared-sensitive camera, which had no viewfinder, was a constant challenge for cameraman Owen Newman.

While makers of infrared-sensitive cameras guard the secrets of their
products, the basics behind the cameras are well understood. The
infrared-sensitive camera picks up reflected light waves much like the human
eye. The lens directs all of the incoming waves to a small internal chip that
is sensitive to visible and near-infrared waves. The chip records the relative
strength of the waves at each one of its 500,000-plus pixel points. It then
changes this grid of energy levels into different shades of gray: the greater
the strength of the near-infrared wave, the brighter the pixel. This
information is then translated to the pixels on a black and white screen. These
infrared-sensitive cameras give us just a first glimpse into night wildlife.
With new technologies developing, we can only imagine what lies in store for us
in the future. Dark, camera, action!